Various types of polyethylenes are known in the art. Low density polyethylene (“LDPE”) can be prepared at high pressure using free radical initiators, or in gas phase processes using Ziegler-Natta or vanadium catalysts, and typically has a density in the range of 0.916–0.940 g/cm3. LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene in the same density range, i.e., 0.916 to 0.940 g/cm3, which is linear and does not contain long chain branching is also known; this “linear low density polyethylene” (“LLDPE”) can be produced with conventional Ziegler-Natta catalysts or with metallocene catalysts. Relatively higher density LDPE, typically in the range of 0.928 to 0.940 g/cm3, is sometimes referred to as medium density polyethylene (“MDPE”). Polyethylenes having still greater density are the high density polyethylenes (“HDPEs”), i.e., polyethylenes having densities greater than 0.940 g/cm3, and are generally prepared with Ziegler-Natta catalysts. Very low density polyethylene (“VLDPE”) is also known. VLDPEs can be produced by a number of different processes yielding polymers with different properties, but can be generally described as polyethylenes having a density less than 0.916 g/cm3, typically 0.890 to 0.915 g/cm3 or 0.900 to 0.915 g/cm3.
U.S. Pat. Nos. 5,272,236 and 5,278,272 disclose polyethylenes termed “substantially linear ethylene polymers” (“SLEPs”). These SLEPs are characterized as having a polymer backbone substituted with about 0.01 long chain branches/1000 carbons to about 3 long chain branches/1000 carbons, more preferably from about 0.01 long chain branches/1000 carbons to about 1 long chain branches/1000 carbons, and especially from about 0.05 long chain branches/1000 carbons to about 1 long chain branches/1000 carbons. As used in U.S. Pat. Nos. 5,272,236 and 5,278,272, “long chain branching” is defined as a chain length of at least about 6 carbons, above which the length cannot be distinguished using 13C NMR spectroscopy. It is further disclosed that the long chain branch can be as long as about the same length as the length of the polymer backbone.
As used in the present disclosure, the term “linear” is applied to a polymer that has a linear backbone and does not have long chain branching; i.e., a “linear” polymer is one that does not have the long chain branches characteristic of a SLEP polymer.
Blends of polyethylenes are also known. Blending has been used to form polymer compositions having altered properties, such as melt index and various processability characteristics. Blending has also been used to form polymer compositions having properties enhanced for particular end uses. For example, polymer blends have been used to form cast or extruded films with altered film properties, such as toughness, tear resistance, shrink properties, and other desired film characteristics.
U.S. Pat. No. 5,972,444 discloses a polyolefin shrink film made from a polymer mixture which includes a first ethylene polymer component having a single differential scanning calorimetry (DSC) melting peak or a single Analytical Temperature Rising Elution Fractionation (ATREF) peak and a second ethylene polymer component having one or more DSC melting peaks, wherein the density differential between the two component polymers about 0 to about 0.03 g/cm3. Preferably, the first ethylene polymer component is a substantially linear ethylene polymer (SLEP) and the second component polymer is a heterogeneously branched linear ethylene polymer. Commonly assigned PCT publication WO 98/21276 discloses similar films.
U.S. Pat. No. 5,707,751 discloses shrink film compositions comprising precise combinations of “narrow, substantially singular” melting point polyethylene with a higher melting point (by at least about 10° C.) polyethylene.
U.S. Pat. No. 5,382,631 discloses polymer blends and single or multilayer films made from the blends. The blends are made from components having a narrow molecular weight distribution (e.g., Mw/Mn 3) and a narrow composition distribution (e.g., CDBI>50%). The blend components of the blend can all have the same molecular weight but different comonomer contents, the same comonomer content but different molecular weights, or comonomer contents which increase with molecular weight. The blends have either Mw/Mn>3 and/or CDBI<50%, and combinations of each, and can be bimodal with respect to either or both molecular weight and/or comonomer content. The blends are generally free of blend components having both a higher average molecular weight and a lower average comonomer content than another blend component.
U.S. Pat. Nos. 5,907,943 and 5,922,441 disclose a thermoplastic stretch wrap film containing at least three layers, the inner layer of which comprises a blend of a “low dispersity polymer” and either a high pressure low density polyethylene resin, a very low density polyethylene resin or a combination thereof. The low dispersity polymer has a density of from about 0.88 to 0.94 g/cm3.